Determination and Optimization of the Luminescence External Quantum Efficiency of Silver-Clusters Zeolite Composites Eduardo Coutino-Gonzalez, †,# Maarten B. J. Roeffaers, ‡,# Bjorn Dieu, † Gert De Cremer, ‡,▽ Sven Leyre, §,∥,# Peter Hanselaer, §,∥,# Wim Fyen, ⊥,# Bert Sels,* ,‡,# and Johan Hofkens* ,†,# † Department of Chemistry, KULeuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ‡ Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium § Light & Lighting Laboratory, Catholic University College Ghent, Gebroeders Desmetstraat 1, B-9000 Gent, Belgium ∥ ESAT/ELECTA, KULeuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium ⊥ KULeuven Research & Development, KULeuven, Waaistraat 6, B-3000 Leuven, Belgium # SIM (Flemish Strategic Initiative on Materials), SOPPOM Program, Technologiepark 935, B-9052 Zwijnaarde, Belgium * S Supporting Information ABSTRACT: We have measured for the first time the external quantum efficiency (EQE) of silver clusters containing zeolites (henceforth referred to as silver-clusters zeolite composites). These materials, fabricated by silver cation exchange followed by a thermal autoreduction process, have EQEs up to 69%. Because of their unique spectral features such as large Stokes shift and high EQE, these materials could be potentially used as phosphors for the fabrication of fluorescent lamps and as wavelength convertors in solar cells. An EQE comparison between less pure commercial silver- loaded zeolites and self-synthesized silver-zeolites showed the importance of the chemical and optical purity of the starting host material. Besides this, the zeolite topology and silver content play an important role on the luminescent performance of such materials. The ability to reliably measure the EQE enabled us to further optimize the synthesis of silver-zeolite composites. A new reduction−oxidation cycle is demonstrated not only to improve the luminescent performance of the silver-zeolite composites but also to enhance their water stability. ■ INTRODUCTION Silver-zeolite composites are a versatile family of materials. Their applications range from catalysts, 1 to antibacterial materials, 2 information storage, 3−5 and pressure or chemical sensors. 6 The use of zeolites as molecular scaffolds for the fabrication of luminescent materials by incorporating transition metals has been explored. 7−10 The creation of such oligoatomic metal clusters in zeolite voids is based on a ship-in-a-bottle approach, taking advantage of the high cation exchange capacity of zeolites. One of the most popular methods to produce metal clusters in zeolitic matrices is by exchanging the original charge- balancing cations present in the zeolites with the desired metal ions, followed by a thermal treatment. Small clusters are thus formed, whose size is ideally limited by the cage/pore dimensions of the zeolite topology. The mechanism of cluster formation has been proposed as an “autoreduction” mecha- nism, in which the electrons needed for metal ion reduction are provided by zeolite framework oxygen (resulting in local lattice damages) or by oxygen of the hydration water in the zeolite. 1,11−13 Alternatively, chemical reduction 14 and photo- activation, 15 have been used for the creation of metal clusters and nanoparticles in zeolite matrices. We recently reported the production of luminescent silver- clusters in zeolites. 16 In this study, the effect of zeolite topology, silver loading, and counterion on the luminescence color was evaluated systematically; green and red emitters were mainly found in LTA zeolite, whereas for FAU zeolite, green and yellow emitters were observed. The photo-, chemo-, and hydrostability of the materials were investigated in detail. 15−17 The study revealed dramatic differences among the different types of silver clusters; most of the samples presented a high photo- and chemostability. However, the luminescence performance of certain samples, as qualitatively observed, was highly affected by the level of hydration in the environment. 17 Despite their water sensitivity, these materials have serious potential to be used as phosphor substitutes for the fabrication of fluorescent lamps or as wavelength converters in solar cells 15−19 because of their large Stokes shift and luminescence performance. Next to price, the major decisive factor for such application is the external quantum efficiency (EQE). The EQE Received: January 16, 2013 Revised: March 14, 2013 Published: March 19, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 6998 dx.doi.org/10.1021/jp400511z | J. Phys. Chem. C 2013, 117, 6998−7004